Image sensors include a number of picture elements or pixels. Each pixel obtains one portion of a final acquired image. The pixels are in different physical locations on the semiconductor substrate chip that forms the image sensor. Each of the pixels includes a circuit that individually obtains a portion of the image. Hence, each of these pixel circuits must receive power.
The power is typically applied from the outside of the chip, and shuttled across the chip to each of the pixels. As the power is delivered across the chip, each area over which the power passes effectively forms a resistance. The resistance leads to a voltage drop. Therefore, power delivered to portions which are further away from the spot where the power is sourced may have a lower voltage, as compared with other portions which are closer to the edge of the chip.
It is important to supply consistent amounts of power throughout the whole chip. Any variation in the supply power, e.g., the voltage across the chip, may produce effects in the final image which is obtained by the chip. For example, a pixel circuit which receives a slightly lower voltage may show up as slightly brighter than other pixels or slightly less bright than other pixels. Since this variation will be consistent from frame to frame, the change may be very noticeable.
As the image sensor chips become larger, it becomes more difficult to maintain a constant voltage everywhere on the chip. For example, chips which have 2000-3000 columns may have a voltage drop across the extent of the chip.
Conventionally, a relatively large diameter power supply line is run to attempt to minimize the effects of the voltage drop.